Wide band gap semiconductors have been applied to various types of semiconductor devices, such as power elements (also referred to as “power devices”), environmental resistance elements, high-temperature operation elements, and high-frequency elements. Among others, applications to power devices, such as switching elements and rectifier elements, have been drawing public attention.
Typical switching elements of power devices include field effect transistors, such as metal-insulator-semiconductor field effect transistors (hereinafter, “MISFETs”) and metal-semiconductor field effect transistors (hereinafter, “MESFETs”). Such a switching element can be switched, based on the voltage applied between the gate electrode and the source electrode, between an ON state where a drain current of some A (ampere) or more flows therethrough, and an OFF state where the drain current is zero. In the OFF state, a high peak inverse voltage of some hundreds of V or more is realized.
Among other wide band gap semiconductors, power devices using silicon carbide (SiC) (SiC power devices) have been actively developed because it is relatively easy to manufacture an SiC substrate and it is possible to form silicon oxide (SiO2), which is a high-quality gate insulating film, through thermal oxidation of SiC.
Since SiC has a higher breakdown electric field and a higher thermal conductivity than Si, it is easy, with an SiC power device, to realize a higher peak inverse voltage and a lower loss as compared with an Si power device.
In order to conduct an even larger current through a power device such as a MISFET, it is effective to increase the channel density. Therefore, vertical power MISFETs of the trench gate structure have been proposed, replacing the conventional planar gate structure. The channel region is formed in the surface of the semiconductor layer in the planar gate structure, whereas the channel region is formed on the side surface of the trench formed in the semiconductor layer in the trench gate structure.
However, a problem with MISFETs of the trench gate structure is that the electric field intensity to be applied to the gate insulating film is very high.
To address the problem that the electric: field intensity is high at the terminal portion of stripe-shaped trenches, Patent Document No. 1 proposes narrowing the width of the trench stepwise at the terminal portion and reducing the depth of the trench at the terminal portion in order to reduce the electric field intensity acting on the terminal portion.
On the other hand, Patent Document Nos. 2 and 3 propose making the gate insulating film thicker in a bottom portion of the trench to increase the breakdown electric field in order to prevent the dielectric breakdown due to the electric field concentration in the bottom portion of the trench. Patent Document No. 2 discloses selectively making the insulating film (thermal oxidation film) thicker in the trench bottom portion by using the (0001) carbon face whose oxidation rate is high as the trench bottom surface. Patent Document No. 3 proposes making the insulating film thicker by the thickness of the oxide film in the trench bottom portion by depositing an oxide film inside the trench and then etching the oxide film selectively so as to leave the trench bottom portion.